The development of improved composite materials has led to increased use of these materials in both engineering and non-engineering applications. Wood-based composite materials such as plywood, wafferboard, and oriented strand board are widely used as engineered structural components. Particleboard, medium density fiberboard, hardboard and other similar composites are typically used in non-engineering applications. Epoxy-graphite and other similar elongated fiber-matrix composites have found acceptance in aviation, sports products and in other applications where high strength and light weight are desired.
All materials have some variability in the strength capability of individual pieces which are similarly manufactured. However, this is a much greater problem with composite materials as compared to homogeneous engineering materials such as metals. Composite materials have relatively wide variations in strength due to the composite nature of the products and the difficulty in achieving uniform strength in the bonding used to join the components together. Variations in the feedstocks and other factors make manufacture of uniformly strong and elastic structures from composite elements difficult and costly.
Manufacturers of composite materials suffer significant quality control problems in connection with bonding and continuity of the components used to make up the composite. Epoxy-graphite fiber composites suffer failure at the fiber/matrix bond. Problems also occur with respect to fiber strength, breakage and orientation, all of which affect overall strength and elastic properties of the structural element. Composite materials also suffer common failures and reduced strength capabilities due to poor bonding, voids, cracks and other problems specific to individual pieces of the composite material.
Previously it has been impossible to nondestructively test structural elements made from composite materials to provide reliable indication of the strength properties of the particular element being tested. It has been typical to test a large number of specimens and to arrive at acceptable design loads based upon the performance exhibited by the vast majority of such samples and using appropriate factors of safety. Heretofore it has not been acceptable to use varying strength capabilities for various pieces because of the fact that the strength of each piece could not be accurately assessed without destructively testing.
In the particleboard, fiberboard, plywood, wafferboard and similar industries, there have been little or no way of assuring that individual members have the desired strength capabilities. Manufacturers have typically used non-strength indicators such as visual indentification of separations, cracks, pockets and other defects in order to selectively eliminate structurally insufficient pieces. Test pieces have also typically been removed at random for destructive testing to obtain a general measure of product strength capabilities. Such manufacturing techniques have not reliably indentified internal defects and have further led to significant waste of usable product, both through testing and removal of cosmetically defective but useful product. Accordingly, there has been a significant need for methods and apparatus for non-destructively testing structural elements made of composite materials to provide a reliable indication or prediction of the strength properties of each particular element.
U.S. Pat. No. 3,423,991 to Collins shows an apparatus for ultrasonically inspecting plywood panels for delaminations and other defects. The apparatus rollably imparts ultrasonic vibrations to the panels along one side and uses a sensing element on the opposite side. High rates of ultrasonic attenuation cause the sensor to have reduced reception thus indicating the location of defects. Reflected pulse-echo mode operation is also possible. The Collins' invention does not seek to predict mechanical properties of the plywood panels for engineering use, but instead is used to identify defective areas of the panels.
U.S. Pat. No. 3,888,108 to Brands shows a pavement testing apparatus adapted for use with conrete pavement. The apparatus uses a falling hammer to impart vibrations to the pavement. The intensity of the vibrations are sensed at two disparate points by accelerometers. The attenuation of the vibrations is used as an indicator of the structural integrity of the pavement.
U.S. Pat. No. 3,504,532 to Muenon shows a non-destructive testing system using vibrations perceived at two locations on the test specimen.
These and other prior art patents have not taught reliable methods or apparatuses for non-destructively testing structural elements made from composite materials in order to provide a usable indication of the strength and elastic properties of the individual items being tested.